Continuing advances in information handling technology have led to the requirement for data storage and retrieval systems capable of handling extremely large volumes of information. A proposed method for satisfying this demand is that of optical recording, in particular optical disc recording, which allows recording and accessing of information at very high data rates with a much greater recording density and archivability than is possible with magnetic recording. A highly focussed laser beam is utilized to record and recover information on the optical recording media. The selection and alignment of diode lasers into an optical recording system is discussed by Barolini et al. in I.E.E.E. Journal of Quantum Electronics, 1981, p. 69, and both read and write apparatus are disclosed in British Patent Application No. 2,016,747A.
Many types of recording media have been disclosed for laser writing and these can be divided into two basic sorts: those which require processing after writing and those which can be read immediately after writing. It is the latter type, possessing "direct read after write" capability and commonly known as "DRAW" media, which are of particular interest.
In order to be useful as a light absorbing layer for the recording element, materials must be able to be applied to a substrate in the form of a thin, smooth layer of high optical quality and predetermined thickness and they must absorb at the frequency of the optical source. Various materials have been proposed for the recording media of DRAW systems, including, for example, thin metal films, metal-impregnated polymers and organic dyes. In these cases the laser beam provides a pulse of heat energy to the recording medium which causes a change in surface morphology; i.e., formation of a bump or crater, by ablation, vaporization or melting.
The most common DRAW media are the thin metal films and, of these, tellurium containing mixtures as disclosed in Lou et al., J. Vac. Sci. Technol, 1981, 18, 78, are widely used. However, the preparation of recording elements incorporating tellurium is by a relatively expensive vacuum sputtering technique in which the metal does not adhere well to the substrate. It also presents environmental complications because of its toxicity.
Examples of the use of metal-impregnated polymers in recording elements include the silver-impregnated gelatin systems disclosed in U.S. Pat. No. 4,278,758. Greater sensitivity is claimed for these systems than for the tellurium films, but high concentrations of expensive silver are used in the recording medium.
A possible alternative system uses organic compounds in place of expensive metals. As well as providing advantages of cost, the thermal properties of organic compounds are generally superior since they possess low thermal conductivity and low melting/decomposition temperatures. With the use of such systems it is important that the absorption of dye therein corresponds as closely as possible with the emission of the recording laser. Of the various lasers available, semi-conductor laser dioses have the advantages, over conventional gas lasers, of low cost and size and the possibility of easy signal modulation. The problem is, therefore, one of finding organic materials which have all the requisite physical properties and absorb strongly in the region compatible with laser diodes; i.e., the near infrared region of the spectrum, wavelengths between 700 and 1400 nm. The use of hydroxy-squarylium dye for optical data storage is disclosed in Jipson and Jones, J. Vac. Sci. Technol., 1981, 18, 105, but the dye absorbs strongly only in the visible region of the spectrum. Phthalocyanines are disclosed in European Patent Application No. 79200789 and can readily be vapor coated onto a substrate. However, their sensitivity is reduced by their relatively high ablation temperature (300.degree. to 400.degree. C.) and crystallization is always a danger when a coating of pure dye is used.
Dye-in-polymer systems go some way towards avoiding crystallization. An organometallic complex has also been coated out in a polymer binder. Crowly et al., IBM Technical Disclosure Bull, 24, No. 11B, 1982, reports that a nickel dithiene complex dissolved in a film of polystyrene undergoes ablation-typ holeburning with infrared radiation, but no data has been given on the sensitivity of the system. Law et al., Appl. Phys. Lett., 1981, 39, 718, discloses a thiacyanine dye coated in poly(vinyl acetate) solution without any evidence of the formation of microcrystals. However, the intense monomer absorption band observed in solution was lost when the dye was spread in a polymeric binder and the resultant absorption was significantly blue-shifted and reduced in intensity.
As reported above, dye-polymer systems suitable as optical data storage recording media require near infrared absorbing dyes which are compatible with laser diode emission. Heptamethine and longer chain cyanine dyes are known which have absorption maxima in the near infrared region of the spectrum, together with exceptionally high extinction coefficients. However, simple near infrared absorbing cyanine dyes are unsuitable for use as optical data storage media both from solubility considerations and because, when coated out in polymeric binders, their absorption curve is broadened and shifted hypsochromically as reported in Law et al., Appl. Phys. Lett., 1981, 39, 718.
It is highly desirable that the recording layers of an optical recording element have a small thickness; e.g., below 1 .mu.m and an optical density of about 0.4 or higher. Whilst it has been possible to achieve these desirable criteria with recording layers which consist entirely of dye, heretofore it has not been readily possible to manufacture dye-containing layers of a film-forming polymeric binder which have the desired properties without employing specific binder formulations. In particular, many known dye/binder systems must be used in comparatively large thicknesses of more than 1 .mu.m to be able to realize the sufficiently high optical density of the layer.
One particular dye/binder system which is able to achieve the desired criteria is disclosed in U.S. Pat. No. 4,364,986. This patent discloses an information recording element for optically recording optically readable information comprising a disc-shaped substrate on one surface of which there is present an optically readable servo track which has at least partially a relief structure of servo areas situted alternately at a higher and lower level and on which servo track there is present, as a recording layer, a mixture of a dye and a copolymer (1:1) of methyl vinyl ether and maleic anhydride or a semi-ester of said copolymer and an aliphatic alcohol which may be substituted with an aromatic radical and contains 1 to 8 carbon atoms.
Our copending British Patent Application No. 8237040 provides an optical recording element in which information can be recorded and read directly afterwards by means of laser light, the element comprising, as a recording medium, an effective amount of one or more dyes of the formula: ##STR1## in which:
m+p=2, preferably m=p=1,
n is 0 or 1,
s is 0 or 1,
Z.sup.1 and Z.sup.2 independently represent the non-metallic atoms necessary to complete a heterocyclic nucleus of the type present in cyanine dyes,
R.sup.1 and R.sup.2 independently represent an alkyl, substituted alkyl, alkenyl, substituted alkenyl or aralkyl group of up to 20 carbon atoms,
R.sup.3 and R.sup.4 independently represent a hydrogen atom or an alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkoxy or substituted alkoxy group of up to 10 carbon atoms,
R.sup.5, R.sup.6, R.sup.7 and R.sup.8, which together may not contain more than 12 carbon atoms, independently represent a hydrogen atom, an alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl or alkaryl group, or
one of R.sup.5 and R.sup.6 together with one of R.sup.7 and R.sup.8 represent the necessary atoms to complete a carbocyclic ring (e.g., a benzene ring) in which case the others of R.sup.5 to R.sup.8 are absent,
Q.sup.1 and Q.sup.2 together represent the non-metallic atoms necessary to complete an acidic nucleus of the type present in oxonol or merocyanine dyes.
The elements of that application use a particular highly advantageous class of chain-substituted cyanine dyes. The dyes are advantageous not only in respect of their improved solubility in solvent and binder, but also their spectral characteristic in the near infrared region of the spectrum. The dyes retain a very high absorption in the near infrared when present in a layer in an optical recording element. The combination of infrared wavelength of absorption specificity, high extinction coefficient values and solubility leads to these dyes being particularly useful for laser addressed optical recording elements.